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PH-EP-2013-114

(Submitted on 16 Jul 2013)

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Abstract

Prompt production of charmonium $\chi_{c0}$, $\chi_{c1}$ and $\chi_{c2}$
mesons is studied using proton-proton collisions at the LHC at a centre-of-mass
energy of $\sqrt{s}=7$TeV. The $\chi_{c}$ mesons are identified through their
decay to $J/\psi\gamma$, with $J/\psi\to\mu^+\mu^-$ using photons that
converted in the detector. A data sample, corresponding to an integrated
luminosity of $1.0\mathrm{fb}^{-1}$ collected by the LHCb detector, is used to
measure the relative prompt production rate of $\chi_{c1}$ and $\chi_{c2}$ in
the rapidity range $2.0<y<4.5$ as a function of the $J/\psi$ transverse
momentum from 3 to 20 GeV$/c$. First evidence for $\chi_{c0}$ meson production
at a high-energy hadron collider is also presented.

(left) Ratio of $\chi _{ c 2}$ to $\chi _{ c 1}$ cross-sections at $\sqrt{s}=7$ $\mathrm{\,TeV}$ for $2.0<y<4.5$.
The statistical uncertainty is shown with a red error bar and the systematic uncertainty with a hashed rectangle.
(right) Comparison of the LHCb results (with total uncertainty) with the NLO NRQCD calculation from Ref. [5] (blue shading) and the LO
NRQCD calculation of Ref. [24] (solid green). The LHCb results are obtained assuming the $\chi _{c}$ mesons are produced unpolarized.

Comparison of the ratio of $\chi _{ c 2}$ to $\chi _{ c 1}$ cross-sections obtained by LHCb using calorimetric photons [12] (green open squares),
CMS result [11] (blue filled squares), CDF result (purple filled triangles) [10] and the result presented here (red open circles)
under the assumption (left) of unpolarized states and (right) under the assumption $(m_{\chi _{ c 1} },m_{\chi _{ c 2} })=(0,0)$ in the helicity frame.
The uncertainty due to the limited knowledge of the branching fractions of $\chi _{c} \rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} \gamma$, which is common to all the measurements, is not included here.

Measurements of the ratio of $\chi _{ c 2}$ to $\chi _{ c 1}$ production cross-sections for the given $p_{\rm T}^{ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} }$ range assuming unpolarized $\chi _{c}$ production.
The first uncertainty is statistical, the second is systematic, the third is from the branching fractions used
and the last gives the maximum correction due to the unknown polarization.